The invention relates to a process for the oxidation of hydrocarbons.
More particularly, the invention relates to a process for the oxidation of hydrocarbons with gases containing molecular oxygen and in the presence of boron compounds to form the corresponding alcohol/ketone mixtures. More particularly, the invention relates to a process in which the boron compounds used are completely recovered and reused and water-soluble organic secondary products are eliminated by oxidation.
Hydrocarbons can be oxidized with gases containing molecular oxygen. Of considerable technical significance is the oxidation to form alcohols which, to increase selectivity, are esterified by conventional methods with boron compounds as described, for example, in U.S. Pat. No. 3,243,449.
Typical boron compounds used in oxidation processes of this kind include boric acids such as, for example, orthoboric and metaboric acid, boric acid esters such as, for example, the mono esters of metaboric acid with the alcohol of the hydrocarbon to be oxidized, for example cyclohexylmetaborate when cyclohexane is the starting material, and boric acid anhydrides such as, for example, B.sub.2 O.sub.3 and B.sub.4 O.sub.5. Mixtures of these boron compounds can also be used.
Recovery of the boron compound used is of considerable importance to the economy of this process. This is normally effected by hydrolysing the alcohol-boric acid ester with water to form the corresponding alcohol and orthoboric acid. Following separation of the organic phase, consisting of unreacted hydrocarbon, the alcohol and small quantities of by-products, the orthoboric acid present in the aqueous phase is recrystallized by distilling off water and/or by cooling and, after filtration and washing, is converted into the boron compound suitable for the oxidation process. The residual "boric acid mother liquor" is reused together with fresh water for hydrolysis.
Water-soluble by-products of the oxidation process such as, for example, alcohols, carboxylic acids and hydroxy carboxylic acids, accumulate in this mother liquor. They prevent crystallization of the boric acid and rapidly reduce the yield because they are recycled to the oxidation process with the crystallized boric acid (cf. DOS No. 1,618,514). To prevent this, the concentration of organic by-products in the boric acid mother liquor has to be kept within reasonable limits. This is done by continuously separating and discarding part of the boric acid mother liquor. This is inevitably accompanied by the loss of the quantity of boric acid present in the separated component of the mother liquor which has to be replaced by introducing fresh boron compound. Accordingly, substantially quantitative recovery of this boric acid is desirable and, moreover, necessary for reasons of pollution control.
Hitherto, two processes have been proposed for this purpose:
1. Another crystallization stage is employed in which the mother liquor is reconcentrated. Approximately 70% of the boric acid present in the mother liquor crystallizes out on cooling. This component is returned to the first crystallization stage after filtration and washing for the purposes of recrystallization. This post-crystallization process is described in DOS Nos. 1,618,514 and 1,768,839.
2. The second process for avoiding losses of boric acid, described in Belgian Pat. No. 783,255, is distinguished by the fact that a component stream of the hydrolysis water circuit is freed from organic impurities by extraction with a mixture essentially containing alcohols and ketones, and the extracted hydrolysis water is returned to the hydrolysis water circuit.
Unfortunately, at least 30% of the boric acid is also dissolved both physically and chemically through complex formation in the extractant. This quantity of boric acid cannot be recovered.
Both the aforementioned processes for treating boric acid mother liquor are attended by the disadvantage that a maximum of only 70% of the boric acid can be recovered. In addition, they are unsatisfactory insofar as they do not solve the problem of ultimately eliminating the large quantities of organic secondary products present in the mother liquor, which represents a pollution hazard.